X-ray visualisation of solidification and two-phase phenomena

X-ray radiography is a useful tool for a non-invasive, in- situ visualisation and characterization of gas bubbles in opaque metal alloys or solidification processes. In particular, the simultaneous study of solidification phenomena on different length scales (dendrite networks - microscale and flow structure - mesoscale) is the main advantage of a conventional X-ray radiography.

In MHD department two experimental setups for visualization of solidification and two-phase phenomena in metallic alloys were developed.

Microfocus X-ray source & solidification setup

The solidification experiments were monitored by an X-ray radiographic set-up delivering images with a spatial resolution of a few microns. A microfocus X-ray tube equipped with a tungsten target (phoenix X-ray XS225D-OEM) has been utilized. After passing the solidification cell the attenuated X-ray beam impinges an X‑ray image intensifier (Thales TH9438HX 9”), where the X-rays are converted into a two-dimensional visible light distribution which is recorded by a CCD camera (Kappa CF8/1 BV-3) with a scan rate of 50 half frames per second.

The majority of experiments was performed with a Ga–25wt%In alloy (T Liquidus = 25 °C, T Solidus = 15.3 °C) which was prepared from high-purity Ga (99.99%) and In (99.99%). It was melted in a furnace and filled into a 30 x 30 mm2 Hele-Shaw quartz cell having a 150 µm gap parallel with respect to the direction of the X-ray beam.

Figure 1: Experimental setup for the solidification experiments: sketch of the X-ray diagnostic system and Hele-Shaw solidification cell equipped with electric system and Peltier elements.

The visualization experiments with sufficient spatial resolution (5-10 µm) deliver simultaneous information of the dendrite structure, the concentration fields and the flow patterns especially in the vicinity of solidification front (Fig 2a and 2b). A more detailed analysis of the specific phenomena such as the dendrite sidearm development or fragmentation requires X-ray techniques with a much better spatial resolution (below 1 μm). Synchrotron visualization experiments were performed at the ROBL (BM20) and ID19 beam line at the ESRF in Grenoble at a spatial resolution of < 0.5 µm (ID19) and 2µm (BM20).

ISOVOLT X-ray source & two-phase flow setups

An industrial X-ray tube (ISOVOLT 450M1/25-55 from GE Sensing & Inspection Technologies GmbH) was used for the bubble visualization experiments (Fig. 3a). The high power ISOVOLT X-ray source operating with a maximum voltage of 320 kV and a current of 14 mA generates a divergent polychromatic X-ray beam. A scintillation screen (SecureX HB from Applied Scintillation Technologies) is attached to the surface of the container. The non-absorbed part of the X-ray beam comes upon to this scintillation screen where its intensity is converted into visible light. The further imaging is completed with a lens system (Thalheim – Spezial - Optik) and a high-speed video camera (Pco.edge from PCO) equipped with a sCMOS-sensor.

The experiments were realized at two setups: at the X-LIMMCAST facility (continuous casting model) (Fig 3b) and at a rectangular container (Fig 3c).

Figure 3: Photographs of a) the ISOVOLT X-ray source; b) the experimental setup X-LIMMCAST with lead shielding; c) sketch of a Plexiglas container.